In a typical automobile, many relays are connected through control switches to the battery. Battery voltage can vary widely depending on the condition of the battery, the charging system, load changes and ambient conditions. Power dissipation across an energized relay coil is a function of the square of that voltage and appears as coil heating. Excessive heat can destroy the relay coil and the coil container and the associated electronics that reside with the relay's in a typical electronic module. To the extent that the battery voltage varies and there are tolerances in the power dissipation capacities, it is desirable to maintain the voltage across a relay coil as constant as possible.
A relay coil requires a minimum voltage across its terminals in order to ensure actuation of its switch contacts. Any voltage in excess of this minimum "pull-in" voltage increases the coil power dissipation and is lost in the form of heat. Therefore, it is desirable to regulate the coil voltage to a value greater than the minimum "pull-in" to minimize the power dissipation and heat generation.